Computer systems output data in either monochrome or color formats. In certain applications, the display of data in color has many advantages over the monochrome display of data. The use of color allows for easy identification of certain data on a display. For example, a red field could mean data entered incorrectly, whereas a green field could mean data entered correctly. The use of colors also allows sophisticated multicolor graphs, charts, and diagrams to be displayed and printed. Finally, the use of color in an application has a particular aesthetic appeal to computer users that is similar to the appeal color television has over black-and-white television.
Computer systems typically support a variety of color output devices, including video displays and printers. Each of these output devices has differing characteristics. For example, the IBM 8514/A graphics adapter, which provides an interface between the computer and the display, provides the capability to display over 256,000 colors, but only 256 different colors can be displayed at a time. The IBM Enhanced Graphics Adapter (EGA) can display 64 different colors, but only 16 colors can be displayed at a time. When using these adapters, the program sending data to the adapter must specify which colors are the "active" colors; that is, the colors that currently are selected for display.
Each application program that displays color data must accommodate the differences in the number of active colors the various graphics adapters support. Systems software, such as Microsoft's Windows and Presentation Manager, provides a device-independent application programming interface. A developer of application programs can use standard systems routines to display information on a color output device. The systems software adjusts the data to accommodate the differing characteristics of the graphics adapter, so the application programmer need not be concerned about the differing characteristics of the graphics adapters.
Color graphics adapters normally have three basic color components: red, green, and blue. Each picture element (pel) on a display can be set to any one of the active colors by setting each color component, referred to as a red-green-blue (RGB) value. The intensity of each color can be varied. For example, a low-intensity red value would appear as dim red and a high-intensity red value would appear as bright red. The IBM 8514/A can display 64 different intensity levels of each color, but the IBM EGA can display only 3 different intensity levels for each color.
The IBM 8514/A has 256 active colors. Each active color can be represented in binary form using 8 bits. Each pel has associated with it an 8-bit value representing the active color to be displayed at that pel. By standard programming convention, the 8 bits are divided into 3 bits for red, 3 bits for green, and 2 bits for blue. Thus, eight different intensities of red and green are active, but only four intensities of blue are active. The IBM EGA has only 16 active colors. Thus, each pel has an associated 4-bit value. By standard programming convention, there is one red bit, one green bit, one blue bit, and one intensity bit. The intensity bit selects either high or low intensity for all the colors. Thus, the three colors of a given pel can be displayed either in all high intensity or in all low intensity.
The device-independent application program interfaces provided by systems software can support a much larger number of active colors than is typically supported by graphics adapters that are used on personal computers. For example, the Microsoft Windows program supports over 16 million active colors. An application program using Windows can specify 8 bits of red, 8 bits of green, and 8 bits of blue. Each 8-bit value represents an intensity level of the color between 0 and 255. To display bright red at a pel, the application would select an RGB value of high-intensity red and of zero intensity green and blue, which is represented as (255,0,0). To display half intensity magenta (purple), the application may select an RGB value of (128,0,128), that is, half-intensity red and blue and zero intensity green.
The systems software maps the 8-bit values to the active colors of the graphics adapters. In computer systems using the IBM 8514/A, the systems software maps the three 8-bit values to one 8-bit value and for systems using the IBM EGA, it maps to one 4-bit value.
This mapping results in undesirable effects. An application may specify similar shades of a color using the three 8-bit values. However, the systems software may map the similar, but different, shades to the same active color. For example, the systems software maps the 256 possible intensity levels for green and blue to just 8 intensity levels for the IBM 8514/A. Thus, typically 32 different application-specified intensities are actually displayed at the same intensity.
It would be desirable to have a graphics adapter that would support 256 different intensity levels for each of the three colors. This would alleviate this undesirable effect, but would require sophisticated graphics adapters not affordable by the typical personal computer user.
It would also be desirable to have a system that would effectively increase the active colors for the existing graphics adapters.